1. Field of Endeavor
The present disclosure relates to a motor driving apparatus, and more particularly to a driving apparatus for operating an interior permanent magnet synchronous motor at a speed higher than a rated speed.
2. Background
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.
A permanent magnet synchronous motor (PMSM) driven by a voltage inverter operates at a speed control mode or a torque control mode. The speed control mode serves to drive a hoist load such as an elevator or a crane, or a variable speed load such as a fan or a pump, and the torque control mode functions to drive a traction motor of an electric vehicle.
Generally, an output of a speed controller is provided in a torque command under a speed control mode, and a speed control mode of a permanent magnet synchronous motor (PMSM) includes a torque control mode. As a result, all the speed control mode and torque control mode obtain a current command from the torque command and perform a current control, whereby the PMSM is controlled.
The voltage inverter performs a current control under a limited DC-linked voltage and a limited current condition to track a torque command. However, there is a problem in tracking a torque command due to the limited DC-link voltage and limited current condition, in a case a high speed driving of the PMSM is needed.
FIG. 1 is a block diagram illustrating a driving system of an interior permanent magnet synchronous motor according to prior art, where the system is driven by an inverter embodied by a vector control independently controlling a flux current and a torque current from an instruction torque.
The conventional driving system includes an inverter (101), an IPMSM (102) and a rotor position detector (103) attached to a rotor of the IPMSM.
The inverter (101) receives a command torque to output voltages (Vas, Vbs, Vcs) capable of being driven by the command torque, and the rotor position detector (103) calculates or measures a rotor position or a rotor speed.
The rotor position calculated or measured by the rotor position detector (103) is used for coordinate change by coordinate converters (106, 110), and the rotor speed is used by a current command generator (104).
The current command generator (104) outputs a current command on a synchronous reference frame in response to the command torque, the rotor speed, and the DC-link voltage of inverter. In case of IPMSM, the current command generator (104) generally uses a 2-D look-up table, where the look-up table outputs d and q-axes current commands on synchronous reference frame relative to an entire driving region.
A current regulator (105) serves to control the current commands obtained from the current command generator (104) to output d and q-axes voltages on the synchronous reference frame.
The coordinate converter (106) uses the rotor position information obtained by the rotor position detector (103) to convert an output voltage of a current controller (105) to a voltage on a stationary reference frame.
A voltage limiter (107) uses an inscribed circle of a voltage limit hexagon to convert a voltage of the coordinate converter (106) to a voltage synthesizable by an inverter unit (108). The voltage limit condition of the voltage limiter (107) is determined by the DC-link voltage, and the voltage positioned at an outside of the inscribed circle of the voltage limit hexagon is prevented from being outputted and stays on the inscribed circle of the voltage limit hexagon.
The inverter unit (108) is a voltage type inverter including a power semiconductor such as an IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), and supplies the voltage commands (Vas, Vbs, Vcs) for following a command torque to the IPMSM (102).
A current sensor (109) is interposed between the IPMSM (102) and the inverter (108) to measure a phase current applied to the IPMSM (102), and the current measured by the current sensor (109) is returned as a feedback to the current command generator (104) and the current controller (105) in response to the coordinate conversion of the coordinate converter (110). Voltage Equations on the synchronous reference frame of IPMSM (102) are provided as below:
                              V          ds          r                =                                            R              s                        ⁢                          i              ds              r                                +                                    L              ds                        ⁢                                          ⅆ                                  i                  ds                  r                                                            ⅆ                t                                              -                                    ω              r                        ⁢                          λ              qs              r                                                          [                  Equation          ⁢                                          ⁢          1                ]                                          V          qs          r                =                                            R              s                        ⁢                          i              qs              r                                +                                    L              qs                        ⁢                                          ⅆ                                  i                  qs                  r                                                            ⅆ                t                                              -                                    ω              r                        ⁢                          λ              ds              r                        ⁢                                                          [                  Equation          ⁢                                          ⁢          2                ]            where, a superscript ‘r’ represents a variable on synchronous reference frame, a subscript ‘s’ represents a variable of stationary reference frame, ‘ωr’ represents an angular velocity of rotor, ‘idsr’ and ‘iqsr’ represent respectively stator d and q-axes currents on the synchronous reference frame, ‘Visr’ and ‘Vqsr’ represent respectively stator d and q-axes voltages on the synchronous reference frame, ‘λdsr’ and ‘λqsr’ represent respectively stator d and q-axes rotor fluxes on the synchronous reference frame, Rs, Lds and Lqs represent respectively stator resistance d and q-axes inductances.
A driving limit condition of IPMSM (102) is divided to a voltage limit condition and a current limit condition, and expressed as under:(Vdsr)2+(Vqsr)2≦(Vs,max)2  [Equation 3](Idsr)2+(Iqsr)2≦(Is,max)2  [Equation 4]where, Vs,max defines a size of maximum voltage synthesizable by the inverter (101), and Is,max defines a maximum or rated current of IPMSM (102).
FIG. 2 is an exemplary view illustrating a driving region of the IPMSM of FIG. 1, where A is a curve of a constant torque, and currents on d and q-axes on the synchronous reference frame relative to a constant command torque may have an infinite combination, B is a current limiting condition of inverter as shown in the above Equation 4, and C and D are examples of voltage limit condition in response to rotor speed, as shown in above Equation 3.
The voltage limit condition in the driving region of the IPMSM (102) is changed in response to the rotor speed, where as the rotor speed increases, the size of the voltage limit condition to E direction decreases.
The sizes of d and q-axes currents on the synchronous reference frame controllable by the inverter (101) relative to the constant command torque are determined in a range satisfying both an interior of the current limit condition of B and an interior of the voltage limit condition of C or D. In a case a voltage margin is sufficient, the voltage limit condition is not affected by limiting factors, such that it would be advantageous to track a current command driving a MTPA (Maximum Torque Per Ampere) in terms of efficiency of IPMSM (102).
For example, in case a predetermined torque command of A is given, and a voltage limit condition is given as C, a current command to follow a command torque is determined at F, where F is a driving point for satisfying the MTPA.
However, in a case the rotor speed increases to cause the voltage limit condition to move from C to D, the driving point must move to G capable of maintaining the maximum output torque where F is a current region uncontrollable by the inverter (101). The moving process of current command as described above has a non-linear relationship because an inductance of IPMSM (102) is saturated according to current size.
Therefore, characteristic of IPMSM (102) is measured off-line during driving of the IPMSM (102) to prepare at least two more 2-D look-up tables, and the current command generator (104) of FIG. 1 is made to generate a current command on the synchronous reference frame based on constant torque, driving speed, DC-link voltage. The 2-D look-up table inputs the torque command and flux information to generate d and q-axes current commands on the synchronous reference frame. At this time, the flux information may be obtained by dividing the DC-link voltage by rotor speed.
A feedback current of the current command generator (104) and the coordinate converter (110) of FIG. 1 is inputted to the current limiter (105). The current limiter (105) is a proportional and integral controller to synthesize an output voltage as per the following Equations.
                              V          ds                      r            *                          =                                            (                                                K                  pd                                +                                                      K                    id                                    s                                            )                        ⁢                          (                                                i                                      ds                    -                    ref                                    r                                -                                  i                  ds                  r                                            )                                -                                    ω              r                        ⁢                          λ              qs              r                                                          [                  Equation          ⁢                                          ⁢          5                ]                                          V          qs                      r            *                          =                                            (                                                K                  pd                                +                                                      K                    iq                                    s                                            )                        ⁢                          (                                                i                                      qs                    -                    ref                                    r                                -                                  i                  qs                  r                                            )                                -                                    ω              r                        ⁢                          λ              ds              r                                                          [                  Equation          ⁢                                          ⁢          6                ]            
The coordinate converter (106) converts an output voltage of the current limiter (105) on the synchronous reference frame to a voltage on the stationary reference frame using the following Equations.Vdss*=Vdsr*cos θ−Vqsr*sin θ  [Equation 7]Vqss*=Vdsr*cos θ+Vqsr*sin θ  [Equation 8]
The voltage limiter (107) limits a voltage of the coordinate converter (106) and outputs the voltage, so that a voltage command can exist within the inscribed circle of the voltage limit condition expressed by a hexagon on the stationary reference frame, and the inverter unit (108) synthesize a voltage of the following Equations from the voltage limiter (107) and supplies the voltage to the IPMSM (102).
                              V          as                =                  V          ds          s                                    [                  Equation          ⁢                                          ⁢          9                ]                                          V          bs                =                                            -                              1                2                                      ⁢                          V              ds              s                                +                                                    3                            2                        ⁢                          V              qs              s                                                          [                  Equation          ⁢                                          ⁢          10                ]                                          V          cs                =                                            -                              1                2                                      ⁢                          V              ds              s                                +                                                    3                            2                        ⁢                          V              qs              s                                                          [                  Equation          ⁢                                          ⁢          11                ]            
Current sensors (109a-109c) measure a phase current between the inverter unit (108) and the IPMSM (102). The coordinate converter (110) converts the phase current to a current on the synchronous reference frame using the following Equations and provides the current to the current limiter (105) as a feedback.
                              i          ds          s                =                                            2              ⁢                                                          ⁢                              i                as                                      -                          i              bs                        -                          i              cs                                3                                    [                  Equation          ⁢                                          ⁢          12                ]                                          i          qs          s                =                                            i              bs                        -                          i              cs                                2                                    [                  Equation          ⁢                                          ⁢          13                ]                                          i          ds          r                =                                            i              ds              s                        ⁢            cos            ⁢                                                  ⁢            θ                    +                                    i              qs              s                        ⁢            sin            ⁢                                                  ⁢            θ                                              [                  Equation          ⁢                                          ⁢          14                ]                                          i          qs          r                =                                            -                              i                ds                s                                      ⁢            sin            ⁢                                                  ⁢            θ                    +                                    i              qs              s                        ⁢            cos            ⁢                                                  ⁢            θ                                              [                  Equation          ⁢                                          ⁢          15                ]            
However, there is a problem in that performance of the IPMSM driving system of FIG. 1 deteriorates, because the current command generator (104) uses a pre-measured (off-line) look-up table to cause subject parameters of the IPMSM to change.
Furthermore, there is another problem in that, even if the subject parameters of the IPMSM are not changed, the driving performance of motor is determined by performance of the look-up table, because the look-up table determines the performance of an entire driving region.
There is still another problem in that a voltage utilization rate of the inverter decreases to thereby decrease the output torque, because amount of voltage synthesized by the inverter is limited by the inscribed circle of the voltage limit hexagon.
It is, therefore, desirable to overcome the above problems and others by providing an improved apparatus for operating the interior permanent magnet synchronous motor.